When Leon Black on Thursday talks to Wall Street on the quarterly conference call of his Apollo Global Management, he may be forced to address a topic that’s seldom been an issue at the high-flying private equity firm: the depth of his bench.

Three of the firm’s 21 senior private equity partners have left or are planning to leave this year, The Post has learned.

All three — Jordan Zaken, 39, who has been at Apollo since 1999, Stan Parker, 38, a telecom expert, and Ali Rashid, 37, a former Goldman banker — were listed in the offering memorandum for the $17.5 billion fund Apollo finished raising at the end of last year, which means their departures were not totally expected.

Since 2008, 44 percent of Apollo’s 27 private equity partners under the age of 50, including the three, have either left or are planning to leave the firm, sources and company documents show.

The number of departures isn’t staggeringly high, but is unusual for a firm that has boasted its turnover is among the lowest in the industry.

Apollo — as recently as last year when marketing its new fund — told investors it had not experienced excessive turnover among its senior private equity professionals.

Rival KKR has had a 38 percent turnover rate among similar private equity partners during a slightly longer time period, and the Blackstone Group has registered a 36 percent rate.

“There’s normal attrition, and then there’s excess senior turnover, and I think this is excess senior turnover,” a source close to the situation said.

An Apollo lawyer countered, saying, “There is nothing unusual about these departures” and denied there is any generational shift. The departing partners have been replaced by promoting or hiring 14 new partners.

At the same time, Black’s two fellow managing partners have ramped up their outside interests of late. Josh Harris, 49, has acquired sports teams — the Philadelphia 76ers and the New Jersey Devils — while Marc Rowan, 51, is investing his personal fortune through a family office.

Rowan has hired former Goldman high-yield, co-head Ken Glassman to help him invest personally in companies, including Beats Music.

In addition, in March it was announced that President Marc Spilker, 49, who used to host the quarterly conference calls, had resigned. Spilker’s departure is why Black will play host on the call Thursday.

The turnover comes as Apollo is in perhaps its best stretch in years — generating enviable returns and raising the biggest post-recession fund.

The open question is whether Black has the team in place to continue Apollo’s success, said the source close to the situation.